Tumor, including leukemia, is one of the major diseases causing human clinical death. The mortality rate of malignant tumors is extremely high in lung cancer, gastric cancer, breast cancer, pancreatic cancer, liver cancer, intestinal cancer and esophagus cancer. So far, there are still no effective therapeutic drugs or methods that can completely eradicate or cure cancer. There is an urgent need for high-quality anticancer drugs with good specificity, high activity, low toxicity and none drug resistance in clinical applications. The incidence, development, metastasis and deterioration of cancer are related to many factors. The abnormality of signal transduction cascades in normal cells, especially that of the multi-functional signal transduction pathways mediated by transmembrane receptor, is one of the major factors leading to cell transformation and cancerization. Protein tyrosine kinases (PTKs) are enzymes that catalyze the phosphorylation of tyrosine residues of proteins and are necessary for multi-physiological functions of cells such as growth, development, differentiation, metabolism, aging and apoptosis. In general, PTKs can be classified into two categories: membrane receptor and cytoplasmic PTKs. PTK abnormalities can directly lead to different clinical diseases, for examples, cancers, inflammations, autoimmune diseases, neurological or cardiovascular diseases. After decades of continuous efforts, people have identified many PTKs, such as EGFR, HER2/3/4, VEGFR, PDGFR, Met, IGF-1R, FGFR, CSF-1R, Trk receptor, Ephrin receptor, TAM receptor, Tie-2, FLT-3, RET, ALK, BCR-ABL, JAKs, SRC, FAK, BTK, SYK and BLK, that can be as drugable molecules for different diseases clinically. Some of such PTK inhibitors have been successfully applied in clinical practice and have demonstrated good therapeutic effects.
EGFR is a member of the EGFR family that includes four transmembrane receptor protein tyrosine kinases: EGFR (HER1/ErbB1), HER2/ErbB2, HER3/ErbB3 and HER4/ErbB4. EGFR family kinases mediate important multiple signaling pathways in cells. They can control and regulate many physiological functions of cells. Basic science researches, big genomic and clinical data indicate that genetic abnormalities of EGFR, HER2, HER3 and HER4, such as point mutation, deletion, amplification and overexpression may not only directly lead to cell malignant transformation and tumorigenesis, but also they are closely related to the proliferation, invasion, survival, metastasis, infiltration, angiogenesis and drug resistance of tumor cells.
In clinical practice, EGFR abnormal genetic variations (overexpression, point mutation, deletion, insertion, etc.) often present in patients with different cancers, especially lung cancer. Lung cancer is a kind of malignant solid tumor with extremely high death rate.
Non-small-cell lung carcinoma (NSCLC), mainly including adenocarcinoma, squamous-cell carcinoma and large cell carcinoma, accounts for about 80% of the entire lung cancer family, and high-frequency variation of EGFR often occurs in NSCLC, leading to the constitutive activation of the signaling pathways mediated by it and cell cancerization. Similarly, the genetic abnormalities of HER2/ErbB2 (such as mutation, amplification and overexpression) occur in patients with NSCLC, especially the patients showing amplification and/or overexpression of HER2/ErbB2. Besides NSCLC, the aberrations of HER2/ErbB2 frequently occur in many other cancers, some of which are even up to over 30%, such as breast cancer (20%), gastric cancer (22˜25%), esophagus cancer (10˜25%), pancreatic cancer (2˜30%), bladder carcinoma (5˜15%), salivary duct carcinoma (15˜37%), cervical cancer (1˜21%), malignant glioma (7˜15%), followed by NSCLC (5%), colorectal cancer (2˜3%), ovarian cancer (6 7%), head and neck cancer (3%), hepatocellular carcinoma (2.4%) and melanoma (0-5%). In addition, HER2 amplification and/or overexpression degree is not only positively correlated with the malignancy grade of the tumor, but also associated with acquired drug resistance of many chemotherapeutics, such as Paclitaxel/Oxaliplatin.
EGFR and HER2 have been used as drugable targets for the development of anti-cancer drugs. Up to now, a variety of new anti-cancer drugs have been successfully developed or developing, such as macromolecular monoclonal antibodies, including Cetuximab, Panitumumab and Herceptin, which target on the extracellular domain of EGFR/HER2 protein molecules, and small molecule inhibitors, such as Gefitinib, Erlotinib and Lapatinib, which work on the intracellular kinase domain of EGFR/HER2 protein molecules. They have been applied clinically for years, and have achieved good therapeutic effects. Like many other anti-cancer drugs, however, EGFR drugs also have the issue of acquired drug resistance. For example, the clinical acquired drug resistance of Gefitinib and Erlotinib or Lapatinib is up to 50%. A variety of factors can cause acquired drug resistance. Among them, the structureal change of the targeted protein molecule is a significant cause. The core structure of the first-generation EGFR inhibitor compounds applied in clinical practice is 4-anilinoquinazoline, which can combine with the active part of EGFR protein kinase, and inhibit the activity of protein kinase by competing with ATP. Genomic DNA mutation often leads to changes of protein amino acid sequence and protein structure conformation. For example, EGFR protein kinase may become constitutively active due to the protein structural changes caused by EGFR exon 19 delation, L858R point mutation of exon 21 or other mutations such as G719S, G719A, G719C, L858R, L861Q and S768I, which are PTKi-sensitive mutants and have comparatively enhanced the inhibiting effects of Gefitinib and Erlotinib on EGFR. Some variations of exon20, however, often lead to drug resistance. For example, when threonine 790, a gate-keeper in EGFR protein kinase domain, is mutated into methionine, it will significantly increase the affinity between the mutanted protein kinase and ATP. The first-generation EGFR inhibitors have lost the competitive capacity with ATP which has led to drug failure and drug resistance. Due to such acquired resistance, the first-generation EGFR inhibitors show no therapeutic effect on 40-55% of NSCLC patients in clinical practice. Studies have also shown that different amino acid insertions in EGFR exon 20 can also confer drug resistance. Although the second-generation irreversible inhibitors, such as Afatinib and Neratinib, covalently binding with the cysteine 797 (Cys-797) in EGFR protein have been developed based on the structure of the first-generation EGFR inhibitors and they present certain inhibitory activity for EGFR T790M in vitro, they still show strong inhibition effects on wild type EGFR and present high adverse events and toxicity clinically. In addition, since they showed no obvious advantage on treatment of NSCLC patients with EGFR T790M expression when solely used, their clinical applications have been greatly limited. Moreover the second-generation EGFR inhibitors have also generated different degrees of acquired resistance which are related to new EGFR mutants and partially other oncogene abnormalities (such as Met/HER3 amplification, PIK3CA/BRAF mutation, NF1 loss and FGFR signaling activation).
Recent researches have shown that small molecule compound WZ4002 with 2,4-pyrimidine as the new core skeleton can work on EGFR T790M mutant with high activity while the effects on wild type EGFR are relatively weak. The early clinical trail data of the compounds CO-1686 and AZD9291 with 2,4-pyrimidine core skeleton successively developed by two pharmaceutical companies show that they have better response to patients with EGFR T790M mutation and relative low side effects. They are a new generation of effective EGFR T790M mutant inhibitors.
Inhibition of EGFR activity can effectively inhibit the growth of NSCLC, however, abnormal expressions of other genes, such as amplification and overexpression of HER2/ErbB2, amplification of HGFR (MET), as well as the amplification and rearrangement of anaplastic lymphoma kinase (ALK) are also closely related to the malignant growth and drug resistance of NSCLC. And in many other malignant tumors, such as gastric cancer, breast cancer, esophagus cancer and salivary duct carcinoma, HER2/ErbB2 is another important cancer target as well. Although anti-HER2 monoclonal antibody drug Herceptin and small molecule compound Lapatinib in the market have clinically presented favorable therapeutic effects, their problems such as acquired drug resistance and blood brain barrier, however, have limited their clinical wildly applications.